The presen invention relates to the transfer of an amount of liquid from one container to another, and more particularly, to a method for ascertaining the overall quality and integrity of a liquid aspiration process using a vacuum actuated pipette.
Fully automated diagnostic analyzers are commercially available to perform chemical assays and immunoassays of biological fluids such as urine, blood serum, plasma, cerebrospinal liquids and the like. Generally, reactions between an analyte in a patient sample and reagents used during the assay, result in generating some sort of signal that can e measured by the analyzer. From this signal the concentration of analyte in the patient sample may be calculated. Such automated analyzers generally use an aspirating means such as a sampling tip, or probe or needle, to transfer predetermined volumes of liquid samples or liquid reagents between receptacles, such as between sample containers, reagent containers and reaction cuvettes disposed on the analyzer. Hereinafter, variations of the term aspirate refer to all of such processes for extracting liquid from one container and depositing at least some of the liquid into the same or another container and further includes the supporting devices required to complete the liquid handling operations.
Aspirator are typically employed for transporting liquids between reservoirs which hold liquid samples and/or reagents and for transporting liquids from such reservoirs to test sites such as in assay cuvettes or cartridges to conduct various tests. The aspirating means typical includes an elongated, needle-like member called a pipette having a hollow passage whereby liquid may be aspirated into and/or dispensed from the sample probe using appropriate pumping resources. The pipette is carried by a transport mechanism adapted to provide horizontal and vertical movement so as to enable the pipette tip to be lowered into a liquid in a reservoir for aspiration of the liquid, and for transporting the liquid to a test site whereat the pipette is lowered to an optimal position for dispensing the liquid. Some type of device, such as a piston assembly, which may be incorporated into the pipette, is operated electronically to aspirate liquid into the pipette and to dispense liquid from the pipette.
It is desirable, when aspirating liquid, to accurately determine the extent any abnormalities or non-uniformities within the liquid adversely affect the overall quality of the aspiration process. Non-uniformities such as clots, bubbles, foam, insufficient volume, etc, are found in many liquids, particularly when the liquid is one of several body fluids being analyzed as these frequently are of non-uniform composition. Various methods have been developed to detect the effect of such non-uniformities on the aspiration process.
U.S. Pat. No. 6,022,747 discloses a blood clot detector having a pressure transducer on an aspiration line to provide output voltage data to a microprocessor corresponding to the vacuum level during aspiration. The microprocessor integrates the vacuum readings over time during the aspiration cycle to provide a pressure integral for each test sample aspiration. A pressure integral is determined for an unclotted aspiration and is used as a reference for comparison with the pressure integrals of each test sample aspiration to determine whether a blood clot has interfered with the test sample aspiration. A valve is provided across an analytical line and an aspiration line to provide selective communication between the aspiration line and the analytical line or to prevent such communication. Communication between the aspiration line and the analytical line permits transfer of a test sample from the aspiration line to the analytical line if the test sample is considered acceptable for sample analysis. Acceptability of the test sample for analysis is based upon a predetermined difference between the reference pressure integral and each test sample pressure integral.
U.S. Pat. Nos. 5,814,275, 5,622,869 and 5,451,373 relate to an apparatus for detecting obstructions of a flow line. A detector housing has first and second openings into a cavity therein. The flow line is attached to the detector housing establishing a flow path through the first opening, the cavity, and the second opening, respectively. A pressure detector detects changes in pressure within the cavity, indicating the presence of an obstruction. A rigid barrier is disposed near the pressure detector on a side of the pressure detector opposite the flow line, so that when said flow line and pressure detector expand, the rigid barrier does not expand and the pressure detector is compressed.
U.S. Pat. No. 5,540,081 relates to a pipetting apparatus provided with clot detection comprising a nozzle for aspirating a sample. A pressure sensor is connected with the nozzle for measuring pressure in said nozzle. A plurality of pressure difference calculating circuits are connected with the pressure sensor, each for inputting an output of the pressure sensor and obtaining a pressure difference at a different pressure calculation period. A plurality of discriminating circuits each having a different discrimination threshold value determined according to each of the pressure calculation periods are provided. An alarm circuit is included for outputting a clot detection alarm signal when at least one of said discriminating circuits discriminates that the obtained pressure difference exceeds the discrimination threshold value.
U.S. Pat. No. 5,503,036 relates to an obstruction detection circuit for detecting an obstruction of a sample probe of an automated fluid sample aspiration/dispensation device and a method for detecting such an obstruction. In one embodiment, the obstruction detection circuit includes a pressure sensor measuring the pressure in a fluid conduit connecting a pump and to a sample probe orifice. The pressure within the connecting fluid conduit is measured shortly after the start of the aspiration or dispensation of a sample volume by the automated fluid sample aspiration-dispensation device. The pressure within the connecting fluid conduit is again measured after the completion of the aspiration or the dispensation by the pump, and if the pressure has not returned to a predetermined range within a predetermined amount of time, an error condition is reported.
U.S. Pat. No. 5,463,895 discloses provides an apparatus and method of detecting non-homogeneity in a fluid sample, such as the presence of foam or bubbles on the surface of the sample, and/or the presence of clots on the surface or in the bulk of the sample. This method involves determining the ambient air pressure within a pipettor as a baseline reading, aspirating air into the pipettor as the pipettor moves towards a sample in container and monitoring for a pressure change in the pipettor to indicate the surface level of the fluid in said container. The pipettor is immersed in the fluid and a volume of fluid is withdrawn from the container; pressure changes are monitored after aspiration and compared to predetermined normal aspiration pressure windows.
Such prior art liquid aspiration processes are not satisfactory in all instances. For example, most known systems for determining the quality or integrity of an aspiration process depend on measuring only differences in vacuum pressure at different intervals during the aspiration process and comparing these measurement to a range of predetermined satisfactory values. Accordingly, as the state of the art advances and more demands are made on the analyzer""s systems there is a continuing need for liquid aspiration systems that are capable of determining additional information defining the overall quality of the aspiration process. In addition, it is useful to know if the integrity of the aspiration means is defective, for instance, as the result of an unwanted air leak.
It is therefore an object of this invention to provide a liquid aspiration method which is capable of determining the overall quality and integrity of the amount of liquid which has been aspirated into a pipette tip. This and other advantages are accomplished in accordance with the invention by providing a liquid aspiration method which includes a method for determining the quality of the aspirated sample through analysis of the pressure profile generated before, during, and after the aspiration process. Sensed aspiration pressure data used for verification of the overall quality of the aspiration process are recorded for each sample aspiration and analyzed immediately following the aspiration event. Pumping resource motion is started a specified number of reads after data collection has started. The present aspiration method checks the aspiration for undesirable events such as partial or complete clogs, or aspiration of air by employing three separate aspiration tests including a pressure difference test to verify liquid was aspirated, a pressure recovery test to check for clogs and aspiration of unwanted cells, and a pressure shape test to check for abnormalities during aspiration, such as clogs, air aspiration, density changes (due to aspiration of blood cells), etc. Three algorithms are employed, and each must produce a positive result for the sample to be released for transfer elsewhere.